The software system SILVIA is a tool for simulation of the duel between Air Defence and Ground Attack Aircraft to be used for studies on a technical-tactical level. In the late nineties a decision was made to introduce a new objectoriented design for the simulation model in SILVIA. Among the reasons for this was to make it possible to introduce new functions which will be necessary for studies of future Air Defence systems. The program structure has then been changed considerably. This report is partly a description of the new objectoriented approach to the design and partly a description of the new implementation of the simulation model in SILVIA.